Cultured human endothelial cells (EC) can present antigen to resting memory T cells and provide sufficient co-stimulation to induce IL-2 and IFN-gamma synthesis. In addition, EC signals induce CsA resistance in T cells, which suggests a mechanism to explain the chronic immune inflammation that often develops in the vessel walls of transplanted organs. In the previous funding period we showed that EC lie along a spectrum of antigen presenting ability between dendritic cells and activated B cells at one end, and smooth muscle cells at the other. EC provide co-stimulation early in T cell activation due to their ability to induce lipid raft aggregation in responding T cells, thus amplifying TCR signaling; they do not appear to target a single, specific co-stimulatory pathway. At later times, however, co-stimulation via specific pathways can alter the ensuing immune response. For example, we have shown that activated EC express the novel B7-like co-stimulatory molecules B7-H1 and GL-50, which have previously been shown to induce IL-10 synthesis. Also at later times in the presence of CsA, EC co-stimulatory signals allow build-up in the T cell nucleus of NFAT, a crucial regulator of IL-2 transcription. This appears to involve down regulation of GSK-3beta as a result of EC-derived wnt-5a signaling. We have identified other wnts in EC, and frizzled receptors in T cells, however, their function in regulating immune responses is not known. We propose to test the hypotheses that: 1) expression by EC of B7-H 1 and GL-50 allows EC to shape a local immune response by altering the balance of cytokines expressed by T cells; and, 2) that EC expression of wnt proteins, and activation of the wnt pathway in T cells, prolongs NFAT nuclear localization thus contributing to CsA-resistance in T cells. Our specific aims are: 1) To test the importance of B7-H 1 and GL-50 as EC co-stimulatory molecules; 2) to determine the role of the wnt pathway in modulating T cell responses to EC; and, 3) to test our hypotheses in vivo using the hu-PBL-SCID mouse grafted with synthetic microvessels. Completion of these aims will provide us with a better understanding of the role of EC in shaping immune responses, and may open avenues for design of therapies that reduce the severity of graft arteriosclerosis.